1. Field of the Invention
The present invention relates generally to an arrayed-waveguide grating type optical multi-demultiplexer for use in multiplexing or demultiplexing a wavelength division multiplexing optical signal, and more particularly to an optical multi-demultiplexer having wide-band optical wavelength characteristics.
2. Description of the Related Art
In the field of optical communications, research attempts have been made to develop a wavelength division multiplexing transmission system that aims at increasing an information capacity by carrying a plurality of signals on light components with different wavelengths and transmitting them through a single optical fiber. In this transmission system, an optical multi-demultiplexer plays an important role in multiplexing or demultiplexing the light components with different wavelengths. Of a variety of types of optical multi-demultiplexers, an optical multi-demultiplexer using an arrayed waveguide grating (AWG) is a promising one since the number of multiplexed wavelengths can be increased with narrow wavelength intervals or spacings.
Examples of the arrayed waveguide grating are disclosed in Jpn. Pat. Appln. KOKAI Publication No. 7-333447, Jpn. Pat. Appln. KOKAI Publication No. 9-297228 and Jpn. Pat. Appln. KOKAI Publication No. 10-197735.
A conventional arrayed waveguide grating comprises input waveguides formed on a substrate; output waveguides; a channel waveguide array composed of a number of curved waveguides; an input fan-shaped slab waveguide formed between the input waveguides and the channel waveguide array; and an output fan-shaped slab waveguide formed between the channel waveguide array and the output waveguides. The channel waveguide array is formed such that the optical path lengths of the curved waveguides are gradually increased from the inside toward the outside of the curved configuration of the curved waveguides thereof so that the adjacent waveguides may have predetermined differences in optical path length.
Thus, wavelength division multiplexing optical signals, which have been incident on input-side ends of the respective waveguides of the channel waveguide array, propagate to output-side ends of the waveguides, while optical phase differences are occurring among the signals in accordance with the frequencies of the signals. The amount of phase difference varies depending on the wavelength of light, and the wavefront of the converged beam is tilted in accordance with its wavelength. Consequently, the respective positions of convergence of light beams in the output fan-shaped slab waveguide vary depending on the respective wavelengths of the light beams. Demultiplexed beams with different wavelengths are converged on output waveguides at different positions in accordance with the respective wavelengths.
If the refractive index of the fan-shaped slab waveguide is uniform and a Gaussian-type input field distribution is provided, as in the prior art, then a Gaussian-distribution type field distribution is accordingly produced at convergence points. As a result, wavelength characteristics have a single peak at a central wavelength of each channel, as in the prior art.
FIG. 10 shows an electric field distribution at a boundary between the output fan-shaped slab waveguide and the output waveguides in the above-described conventional arrayed-waveguide grating. This electric field distribution has a sharp peak at a center of the beam. Thus, if a light component with the peak of electric field can exactly be made incident on the center of a predetermined output waveguide, light can be transmitted with high efficiency.
FIG. 11 shows wavelength characteristics of the conventional arrayed-waveguide grating. In FIG. 11, the abscissa indicates the wavelength, and the ordinate indicates the loss. As shown in FIG. 11, the conventional arrayed-waveguide grating has parabolic wavelength characteristics having peaks at central wavelengths of the respective waveguides.
Consequently, the conventional arrayed-waveguide grating has the following problem. If the wavelength of a laser light source varies even slightly from a central optical wavelength of each waveguide due to, e.g. a temperature variation, the optical loss would considerably increase.
This problem can be solved to some degree by an arrayed-waveguide grating 1 shown in FIG. 12. The arrayed-waveguide grating 1 includes parabolic portions 4 between input waveguides 2 and a fan-shaped slab waveguide 3. Each parabolic portion 4 has such a quadratic-functional shape that the core of the associated input waveguide 2 widens gradually toward the fan-shape slab waveguide 3. A channel waveguide array 5 is connected to the slab waveguide 3.
In the above-described prior art, each parabolic portion 4 provides multiple modes to light that propagates from the associated input waveguide 2 to the slab waveguide 3, and a high-order mode is produced. Thereby, a field distribution with double peaks is created, and wide-band characteristics of a certain level can be obtained.
However, in the arrayed-waveguide grating 1 having the parabolic portions 4, the optical path length increases by a degree corresponding to the parabolic portions 4, leading to an increase in size of the whole structure. In addition, the parabolic portions 4 formed on the adjacent waveguides 2 are situated close to each other. Consequently, when the arrayed-waveguide grating 1 is manufactured, it is difficult to sufficiently bury a clad layer among the cores of the parabolic portions 4.